The synthesis of formic acid from either carbon monoxide and water or carbon dioxide and hydrogen is thermodynamically highly unfavorable and is not a practiced industrial approach to formic acid synthesis. In an effort to overcome these limitations, formic acid has been produced industrially by a number of indirect methods, requiring complicated multi-step reaction and separation sequences, with high capital and energy costs (Reutemann, W.; Kieczka, H., “Formic Acid”, in Ullmann's Encyclopedia of Industrial Chemistry, Volume 16, 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, pp. 13-33). One such method is the acidification of formate salts, produced for example, as a by-product of the Cannizzaro reaction for the production of formaldehyde/aldehyde-based polyols such as trimethylolpropane. This method results in the generation of a salt such as sodium sulfate, with corresponding disposal issues.
The production of formic acid by hydrolysis of formamide, formed by a three-step process involving carbonylation of methanol to methyl formate, reaction with ammonia to produce formamide, followed by hydrolysis-salt formation, is another commercial approach to formic acid synthesis. However, the consumption of ammonia and sulfuric acid, along with the unavoidable production of ammonium sulfate, have made this process economically inferior.
The predominant industrial process for the production of formic acid is via base-catalyzed carbonylation of methanol to methyl formate, followed by hydrolysis of methyl formate to methanol and formic acid, and subsequent separation of water, methanol, and unreacted methyl formate from liberated free formic acid. This process suffers from many drawbacks. Although readily carbonylated, methanol conversion is relatively low (typically less than 30%) at economically viable temperatures and CO pressures. Methyl formate must be distilled from unconverted methanol and the active base catalyst without reversion of the formate back to CO and methanol. The hydrolysis of methyl formate is thermodynamically unfavorable, resulting in only 25-30% conversion, even at a molar ratio of water/ester of 2/1 or higher. If conventional distillation is used to process the hydrolysis reactor effluent, all of the unreacted methyl formate and by-product methanol must be distilled overhead, since formic acid is higher boiling than methyl formate and methanol. The low conversion leads to high recycle ratios of unreacted methyl formate to produced formic acid (typically 3-4 tons of recycled methyl formate per ton of formic acid produced).
Distillation of a hydrolysis reactor effluent (with methanol and methyl formate taken overhead) also produces an underflow that is relatively dilute formic acid in water (typically less than 40 wt. % formic acid), and since formic acid and water form a maximum-boiling azeotrope, the separation of formic acid from water is a relatively energy and capital intensive endeavor. The conventional approach to formic acid-water separation is pressure-swing distillation, whereas the composition of the water/formic acid azeotrope varies fairly significantly with pressure. For example, the water/formic acid wt/wt ratio is about 42/58 at 0.04 bara, 40/60 at 0.067 bara, 25/75 at 1.013 bara, 20/80 at 2.03 bara, 17/83 at 3.04 bara, and 16/84 at 4.05 bara. In the pressure swing distillation system, water is distilled overhead in a first high pressure column, and the maximum boiling formic acid-water azeotrope is taken as a bottoms product. The azeotrope is then further distilled in a second low pressure column with typically 90-99% formic acid as distillate, and a new maximum-boiling azeotrope composition taken as underflow, which is recycled to the high-pressure column. Thus, all the water entering the two-column system with the feed eventually exits as part of the distillate in the high-pressure column. Although the distillate from the high-pressure column can be used for heat integration purposes (either to generate steam upon condensing or directly as a condensing heat source), this pressure swing distillation is very energy intensive, requiring typically 3 to 5 tons of steam per ton of formic acid produced as high purity formic acid.
Several methods have been proposed to improve both the hydrolysis conversion of methyl formate to formic acid and methanol and the separation of formic acid-water. Extraction processes have been proposed for improving the energy consumption for separating formic acid and water, as for example using secondary amides (Hohenschutz et al, U.S. Pat. No. 4,217,460, Aug. 12, 1980, and Wolf et al, U.S. Pat. No. 4,326,073, Apr. 20, 1982). These processes introduce new contaminants into the system and require relatively high vacuum distillation and high energy consumption to separate formic acid from the extractant. Hohenschutz et al, proposed adding a tertiary nitrogen base (U.S. Pat. No. 4,218,568, Aug. 19, 1980) or weak base formamide derivative (U.S. Pat. No. 5,206,433, Apr. 27, 1993) directly to the hydrolysis reaction mixture to shift the equilibrium conversion by forming a salt of the base and formic acid. Such methods also require considerable energy to decompose the salt and liberate formic acid. Buelow et al (U.S. Pat. No. 4,076,594, Feb. 28, 1978) discloses the use of basic extractive distillation agents for separation of formic acid and water, but water must still be distilled overhead and the base-formic acid complex decomposed, resulting again in high energy usage.
Conventional carbonylation-hydrolysis processes utilizing methanol as the alcohol and distillative separation techniques or processes utilizing basic extractants, reactants, or extractive distillation agents are capital and energy intensive. Thus, there is a need for lower energy process for synthesis of formic acid which overcomes these deficiencies.